Tracer control method

ABSTRACT

A tracer control method whereby a tracer machining inhibit area is set and tracer machining is performed while skipping the machining inhibit area, the method including presetting a tracer machining inhibit area (TMI) and a safe level (ZL s ) after taking into account the shape of a model (MDL). The method also includes moving a tracer head along the surface of the model to perform tracer machining in an area (TM), inhibiting tracer machining when the tracer head reaches the machining inhibit area (TMI), and raising the tracer head to the safe level (ZL s ). After reaching the safe level the method includes moving the tracer head along the safe level to a boundary of the machining inhibit area after the tracer head reaches the safe level, thereafter performing an approach operation, and resuming tracer machining in an area (TM&#39;) after the approach is completed.

DESCRIPTION BACKGROUND OF INVENTION

The present invention relates to a tracer control method and, moreparticularly, to a tracer control method whereby a tracer machininginhibit area is set and tracer machining is performed while skipping theinhibit area.

A tracer appartus operates by calculating velocity commands alongvarious axes by means of a tracer arithmetic circuit using a deflectionvalue sensed by a tracer head, driving motors for the corresponding axeson the basis of the velocity commands along these axes to transport atool relative to a workpiece, and repeating these operations to machinethe workpiece into a shape identical to that of the model. In tracercontrol of this kind, the tracer head generally is made to trace theentire surface of the model to provide a machined object having a shapewhich is exactly the same as that of the model. There is now arequirement for tracer machining control whereby a portion of the modelis skipped and both sides (or only one side) with respect to the skippedportion are traced. Such tracer machining control is well-suited forapplication to the machining of, say, a propeller having a boss andblades formed around the boss. The reason is that cutting solely thecomplicatedly shaped blade portions of a propeller under tracer controland applying NC control to cut the boss portion, which is difficult totrace because of its steep gradient, makes it possible to shortenmachining time and improve the precision to which the boss portion ismachined in comparison with tracer machining being applied to theentirety of the propeller.

In the prior art, if the portion desired to be skipped has, say, aconcave configuration, the concave portion is flattened by being filledwith clay or the like, the entire surface of the flattened model istraced to provide a machined object, and a concavity is subsequentlyformed in a predetermined portion of the machined object by NC controlto provide the final article. With this method, however, fabricating themodel is a troublesome operation. Moreover, since the skipped portion ismachined to a flat shape by tracer control and is then machined into aconcavity by NC control, forming the concave portion requires twooperations, thereby lengthening machining time.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a tracercontrol method in which a model need not be remade, and in which thetime required to obtain a final article can be shortened.

Another object of the present invention is to provide a tracer controlmethod whereby a portion of a model, e.g., a portion which is difficultto trace, is skipped automatically, with tracer machining being appliedto both sides, or to only one side, with respect to this portion.

The present invention provides a tracer control method which includespresetting a tracer machining inhibit area (skip area) and a safe level,monitoring a machine position at all times during tracer machining,inhibiting tracer machining when the machine position reaches the skiparea and retracting the machine until the machine position coincideswith the safe level, then transporting the machine in such a manner thatthe machine position moves along the safe level and passes the skiparea, and resuming tracer machining after the skip area is passed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, including 1(A) and 1(B), is a simplified view for describing thepresent invention;

FIG. 2 is a simplified view of a tracing machine tool to which thepresent invention can be applied; and

FIG. 3 is a block diagram illustrating an embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a simplified view for describing the present invention, inwhich (A) is a perspective view and (B) a sectional view. Formed atsubstantially the center of a model MDL is a projection PR. An area TMIcontaining the projection is a tracer machining inhibit area (skip area)in which tracer machining is unnecessary. Two areas TM, TM' on eitherside of the area TMI are tracer machining areas. To trace the model bytwo-way scan tracing, the present invention performs tracing byspecifying the machining inhibit area TMI beforehand, i.e., presettingboundary values XL₁, XL₂ in the feed direction (along the X axis is theillustration of FIG. 1), as well as a Z-axis coordinate ZL_(s)indicative of a safe level in the tracing direction (the Z axis isillustrated in FIG. 1), executing tracing from an approach end point TSTin the direction of the solid arrow line shown in FIG. 1, monitoring themachine position at all times, suspending tracer feed control when themachine position reaches the tracer machining inhibit area, namely whenthe present position of the machine along the X axis becomes equivalentto XL₁, thereafter transporting the machine along the dashed arrow lineuntil the machine position reaches the safe level, namely until thepresent position of the machine along the Z axis coincides with ZL_(s),then moving the machine along the safe level until the present positionof the machine along the X axis coincides with XL₂ (i.e., until theinhibit area is passed), causing the machine to approach the workpieceafter the inhibit area has been passed, executing tracer machining againafter the approach, and thenceforth repeating the foregoing opeations.

FIG. 2 is a simplified view of a machine tool to which the presentinvention can be applied. The tracing machine tool is provided with anX-axis motor XM for driving a table TBL along the X axis, a Z-axis motorZM for driving, along the Z axis, a column CLM mounting a tracer head TCand a cutter head CT, and a Y-axis motor YM for moving the table TBLalong the Y axis. Secured to the table TBL are a model MDL and aworkpiece WK. The tracer head TC contacts and traces the surface of themodel MDL, and the cutter head CT cuts the workpiece WK in accordancewith the shape of the model. As is known in the art, the tracer head TCis arranged to sense deflection ε_(x), ε_(y), ε_(z) along the respectiveX, Y and Z axes of the surface of model MDL, and the deflection ε_(x),ε_(y), ε_(z) along the various axes sensed by the tracer head TC isapplied to a tracer control unit TCC, which performs known tracingcalculations to generate velocity components along respective axes. Forexample, as a tracing method, let us consider two-way scan tracing onthe X-Z plane. This is performed by generating velocity componentsV_(x), V_(z), and applying these to X- and Z-axis motors XM, ZM viaservo circuits SVX, SVZ, respectively, whereby the motors XM, ZM aredriven into rotation. As a result, the cutter head CT is transportedrelative to the workpiece WK to cut the workpiece to the shape of themodel MDL, and the tracer head TC traces the surface of the model. Itshould be noted that SVY denotes a servo circuit for driving a motor YMin order to perform a pick-feed along the Y axis.

FIG. 3 is a block diagram of the present invention for practicing thetwo-way scan tracing shown in FIG. 1. Numeral 100 denotes a known tracercontrol unit, 101 an X-axis control system, and 102 a Z-axis controlsystem. The tracer control unit 100 generates feed velocities V_(x),V_(z) along the respective axes (X and Z axes) on the basis of axialdeflection signals ε_(x), ε_(y), ε_(z) produced by a stylus, andproduces an approach end signal APDEN when the resultant deflectionε(=√ε_(x) ² +ε_(y) ² +ε_(y) ²) surpasses a predetermined value ε_(a).The X-axis control system 101 includes the model MDL, the workpiece WK,the cutter head CT, the tracer head TC, a stylus STL in contact with thesurface of the model, a speed control circuit SCCX, the motor XM fordriving the table TBL along the X axis, which table has the model MDLand workpiece WK mounted thereon, a tachometer TCMX for sensing thevelocity of the motor XM, a pulse generator PGX for generating a singlefeedback pulse P_(x) whenever the motor XM rotates by a predeterminedamount, a ball screw BSR driven by the motor XM, and an X-axis velocitycommand circuit FPGX for transporting the table TBL along the X-axis atthe safe level ZL_(s) and at a predetermined velocity when the presentposition Z_(a) of the machine along the Z axis reaches the safe level.The circuit FPGX includes an arithmetic unit ICC for calculating thedifference (an incremental value) .sub.Δ X between the present positionX_(a) along the X axis and boundary value XL₂ or boundary value XL₁,depending upon the direction of travel (i.e., depending upon the "1","0" logic of a signal MDX indicative of traveling direction, describedbelow), a pulse interpolator PGC, an acceleration/deceleration circuitADC for accelerating or decelerating interpolated pulses, a reversiblecountr ERC for counting up or counting down output pulses from theacceleration/deceleration circuit ADC depending upon traveling direction(where travel is in the positive direction when the traveling directionsignal MDX is logical "1" and in the negative direction when the signalMDX is logical "0"), and for counting up or counting down, dependingupon traveling direction, the feedback pulses P_(x) generated by thepulse generator PGX, thereby to count the difference between the numberof interpolated pulses and the number of feedback pulses, and a DAconverter DAC for producing an analog voltage output which isproportional to the value of the count recorded by the reversiblecounter ERC. SWCX designates an analog gate circuit for providing thespeed control circuit SCCX with the velocity command V_(x) produced bythe tracer control unit 100, or with an X-axis velocity command V_(xc)produced by the X-axis velocity command circuit FPGX, depending upon thepresent position of the machine. XAR denotes an X-axis present positionregister for recording the present position X_(a) of the machine alongthe X axis. The register XAR monitors the present position Xa of themachine along the X axis by counting up or counting down, depending uponthe direction of travel, the feedback pulses P_(x) generated by thepulse generator PGX. The pulse generator PGX actually generated pulsesin two phases, which are displaced from each other by 90°, whenever themotor XM rotates by a predetermined amount. These pulses are applied toa traveling direction discriminating circuit MDDX which produces thetraveling direction signal MDX after discriminating the travelingdirection based on which of the two pulse trains leads the other inphase. XAMC represents an X-axis present position monitoring circuit formonitoring the present position X_(a) of the machine along the X axis.Where the traveling direction is positive (MDX="1"), the monitoringcircuit XAMC produces a control signal GX₁ when X_(a) =XL₁ is true and acontrol signal GX₂ when X_(a) =XL₂ is true. Where the travelingdirection is negative (MDX="0"), the monitoring circuit XAMC producesthe control signal GX₁ when X_(a) =XL₂ is true and the control signalGX₂ when X_(a) =XL₁ is true. The Z-axis control system 102 includes thecolumn CLM driven along the Z axis by the Z-axis motor ZM and having thetracer head TC and cutter head CT mounted integrally thereon, a speedcontrol circuit SCCZ, a tachometer TCMZ, a pulse generator PGZ forgenerating a single feedback pulse P_(z) whenever the motor ZM rotatesby a predetermined amount, and a Z-axis velocity command circuit FPGZfor transporting the column CLM to the safe level (i.e., until Z_(a)=ZL_(s) is established) at a velocity V_(zc) when the present positionX_(a) of the machine along the X axis coincides with XL₁, where tracingis in the positive direction. The Z-axis velocity command circuit PFGZhas the same construction as the X-axis command circuit FPGX. SWCZdesignates an analog gate circuit for providing the speed controlcircuit SCCZ with the velocity command V_(z) produced by the tracercontrol unit 100, or with an Z-axis velocity command V_(zc) produced bythe Z-axis velocity command circuit FPGZ, depending upon the presentposition of the machine. ZAR denotes a Z-axis present position registerfor recording the present position Z_(a) of the machine along the Zaxis. The register ZAR monitors the present position Za of the machinealong the Z axis by counting up or counting down, depending upon thedirection of travel along the Z axis (i.e., depending upon the "1", "0"logic of a traveling direction signal MDZ produced by a travelingdirection discriminating circuit MDDZ), the feedback pulses P_(z)generated by the pulse generator PGZ. ZAMC represents a Z-axis presentposition monitoring circuit for monitoring the present position Z_(a)along the X axis and for producing a control signal GZ when Z_(a)=ZL_(s) holds.

The operation of the present invention will now be described. When theapproach is completed to position the stylus STL at a first point TST(the approach end point in FIG. 1), the tracer control unit 100 performsknown tracer processing in accordance with the stylus deflections ε_(x),ε_(y), ε_(z) to calculate and deliver the feed velocities V_(x), V_(z)along the X and Z axes, respectively. V_(x) and V_(z) enter the speedcontrol circuit SCCX, SCCZ through the analog gate circuits SWCX, SWCZ,so that the table TBL and column CLM are driven by the X- and Z-axismotors XM, ZM, respectively. As a result, the tracer head TC is movedalong the model MDL to produce new deflections ε_(x), ε_(y), ε_(y). Thetracer control unit 100 executes known tracer computations based onthese deflections to generate the feed velocities V_(x), V_(z). The X-and Z-axis motors XM, ZM are driven by the feed velocities Vx, Vzthrough the speed control circuits SCCX, SCCZ, thereby moving the cutterhead CT relative to the workpiece WK to machine the workpiece. Thus,tracer processing is executed in successive fashion based on thedeflection of the tracer head TC to calculate the velocity commandsV_(x), V_(z), and the motors for the respective axes are driven totransport the cutter head CT relative to the workpiece WK, whereby theworkpiece WK is machined into a shape identical with that of the modelMDL.

As tracer machining progresses and the present position of the machinearrives at the tracing machining inhibit area, that is, when thecondition X_(a) =XL₁ is established, the X-axis present positionmonitoring circuit XAMC produces the control signal GX₁. In response togeneration of the control signal GX1, the analog gate circuit SWCX sendsthe input to the speed control circuit SCCX to a value of zero toimmediately halt movement of the table, and the analog gate circuit SWCZsupplies the speed control circuit SCCZ with the velocity commandV_(zc), produced by the Z-axis velocity command circuit FPGZ, in placeof V_(z). Further, when the control signal GX₁ is generated, thearithmetic unit ICC of the Z-axis velocity command circuit FPGZcalculates the difference .sub.Δ Z between the Z-axis position ZL_(s) ofthe safe level and the present position Z_(a) along the Z axis byperforming the following operation:

    ZL.sub.s -Z.sub.a →.sub.Δ Z                   (1)

The pulse interpolator PGC performs a pulse interpolation operationbased on .sub.Δ Z to generate .sub.Δ Z-number of interpolated pulses.The acceleration/deceleration circuit ADC accelerates or decelerates theinterpolated pulses, and the reversible counter ERC calculates thedifference between the number of pulses produced by theacceleration/deceleration circuit and the number of feedback pulsesP_(z) generated by the pulse generator PGZ. The DA converter DACproduces the velocity command V_(zc), namely an analog voltage, which isproportional to the above-mentioned difference. As a result, the columnCLM is elevated by an amount corresponding to .sub.Δ Z and arrives atthe safe level. It should be noted that the Z-axis velocity commandcircuit FPGZ, speed control circuit SCCZ, motor ZM, tachometer TCMZ andpulse generator PGZ construct a well-known positioning servo system.

When the safe level is reached, the Z-axis present position monitoringcircuit ZAMC generates the control signal GZ.

When the control signal GZ is produced, the analog gate circut SWCZsevers the connection between the speed control circuit SCCZ and Z-axisvelocity command circuit FPGZ, and between the speed control circuitSCCZ and tracer control unit 100, and the analog gate circuit SWCXdelivers the output signal of the X-axis velocity command circuit FPGXto the speed control circuit SCCX. Meanwhile, in response to generationof the control signal GZ, the arithmetic unit ICC of the X-axis velocitycommand circuit FPGX obtains the difference between the boundary valueXL₂ and the present position X_(a) along the X axis for travel in thepositive (+X) direction (MDX="1"), or the difference between theboundary value XL₁ and the present position X_(a) along the X axis fortravel in the negative (-X) direction (MDX="0"), by performing thefollowing operation (2) or (3), respectively:

    XL.sub.2 -X.sub.a →.sub.Δ X                   (2)

    XL.sub.1 -X.sub.a →.sub.Δ X                   (3)

The pulse interpolator PGC perform a pulse interpolation on the basis of.sub.Δ X to generate .sub.Δ X-number of pulses. Theacceleration/deceleration circuit ADC accelerates or decelerates theinterpolated pulses, and the reversible counter ERC calculates thedifference between the number of pulses produced by theacceleration/deceleration circuit ADC and the number of feedback pulsesP_(x) generated by the pulse generator PGX. The DA converter DACproduces the velocity command V_(xc), namely an analog voltage, which isproportional to the above-mentioned difference. As a result, the tableTBL is moved along the safe level by an amount corresponding to .sub.Δ Xand arrives at the position XL₂ to move beyond the tracer machininginhibit area. It should be noted that the X-axis velocity commandcircuit FPGX, speed control circuit SCCX, motor XM, tachometer TCMX andpulse generator PGX construct a well-known positioning servo system.

When the present position X_(a) of the machine becomes equivalent to XL₂(for tracing in the +X direction), the X-axis present positionmonitoring circuit ZAMC produces the control signal GX₂. In response togeneration of the control signal GX2, the analog gate circuit SWCXsevers the connection between the speed control circuit SCCX and X-axisvelocity command circuit FPCX, and between the speed control circuitSCCX and tracer control unit 100, and the analog gate circuit SWCZconnects the tracer control unit 100 to the speed control circuit SCCZ.

When the control signal GX₂ is generated, the tracer control apparatus100 executes approach processing and produces the velocity commandsV_(z) based on such processing. The velocity command V_(z) is applied tothe speed control circuit SCCZ through the analog gate circuit SWCZ tolower the column CLM at a predetermined speed along the -Z axis, therebycausing the stylus STL to approach the model MDL. When the stylus STLcontacts the model MDL and the resultant deflection ε surpasses thepredetermined deflection ε_(a), the tracer control unit 100 produces theapproach end signal APDEN. In response to generation of the approach endsignal APDEN, the analog gate circuits SWCX, SWCZ connect the tracercontrol unit 100 to the speed control circuits SCCX, SCCZ, respectively,and the tracer control unit 100 executes ordinary tracer processing.

It should be noted that while the foregoing case relates to tracingalong the +X direction, operation would be performed similarly fortracing along the -X direction.

According to the present invention as described above, a portion of amodel is skipped automatically and tracer machining is performed on bothsides of the skipped portion, or on only one side thereof. Thus itbecomes possible to tracer machine one portion and NC machine theremainder. As a result, a portion for which it is difficult to create anNC tape owing to the complicated shape thereof can be tracer machined,while a portion which is not capable of being tracer machined with greataccuracy and at high speed due to the sharp gradient thereof can be NCmachined, thereby shortening machining time and raising machiningprecision.

The present invention is advantageous in that it may be applied to thefabrication of an article such as a propeller, a part of which ispreferably machined by tracer machining and the remainder thereof by NCmachining in view of machining time and precision.

We claim:
 1. A tracer control method for calculating velocity commandsalong respective axes by using a deflection value sensed by a tracerhead, driving motors, which are provided for the respective axes, inresponse to said velocity commands to move a tool relative to aworkpiece, and causing the tracer head to trace a model, said methodcomprising the steps of:(a) presetting a tracer machining inhibit areaand a safe level; (b) monitoring a machine position to sense whether themachine position has reached the machining inhibit area; (c) inhibitingtracer machining when the machining inhibit area is reached; (d)retracting the machine until the machine position coincides with saidsafe level; (e) causing the machine to pass the machining inhibit areaby moving the machine position along said safe level; (f) causing themachine to approach a workpiece after the machining inhibit area ispassed; and (g) executing tracer machining upon completion of theapproach.
 2. A tracer control method for calculating velocity commandsalong respective axes by using a deflection value sensed by a tracerhead, driving motors, which are provided for the respective axes, inresponse to said velocity commands to move a tool relative to aworkpiece, and causing the tracer head to trace a model, said methodcomprising the steps of:(a) presetting a tracer machining inhibit areaand a safe level; (b) monitoring a machine position to sense whether themachine position has reached the machining inhibit area; (c) inhibitingtracer machining when the machining inhibit area is reached; (d)retracting the machine until the machine position coincides with saidsafe level; (e) causing the machine to pass the machining inhibit areaby moving the machine position along said safe level; (f) causing themachine to approach the workpiece after the machining inhibit area ispassed; and (g) executing tracer machining upon completion of theapproach, said tracer machining being performed by scan tracing.
 3. Atracer control method for calculating velocity commands along respectiveaxes by using a deflection value sensed by a tracer head, drivingmotors, which are provided for the respective axes, in response to saidvelocity commands to move a tool relative to a workpiece, and causingthe tracer head to trace a model, said method comprising the stepsof:(a) presetting a tracer machining inhibit area and a safe level, themachining inhibit area lying in a feed direction and the safe levellying in a tracing direction; (b) monitoring a machine position to sensewhether the machine position has reached the machining inhibit area; (c)inhibiting tracer machining when the machining inhibit area is reached;(d) retracting the machine until the machine position coincides withsaid safe level; (e) causing the machine to pass the machining inhibitarea by moving the machine position along said safe level; (f) causingthe machine to approach the workpiece after the machining inhibit areais passed; and (g) executing tracer machining upon completion of theapproach.
 4. A tracer control method for calculating velocity commandsalong respective axes by using a deflection value sensed by a tracerhead, driving motors, which are provided for the respective axes, inresponse to said velocity commands to move a tool relative to aworkpiece, and causing the tracer head to trace a model, said methodcomprising the steps of:(a) presetting a tracer machining inhibit areaand a safe level, said machining inhibit area being specified by twocoordinate values XL₁, XL₂ in the feed direction, and the safe levelbeing specified by a coordinate value ZL_(s) in the tracing direction;(b) monitoring a machine position to sense whether the machine positionhas reached the machining inhibit area; (c) inhibiting tracer machiningwhen the machining inhibit area is reached; (d) retracting the machineuntil the machine position coincides with said safe level; (e) causingthe machine to pass the machining inhibit area by moving the machineposition along said safe level; (f) causing the machine to approach theworkpiece after the machining inhibit area is passed; and (g) executingtracer machining upon completion of the approach.
 5. A tracer controlmethod according to claim 4,wherein during two-way scan tracing whiletracing along one path step (b) comprises deciding that the machininginhibit area has been reached when a present position X_(a) of themachine in the feed direction coincides with XL₁, and step (f) comprisesdeciding that the machining inhibit area has been passed when thepresent position X_(a) coincides with XL₂ ; and wherein while tracingalong a return path step (b) comprises deciding that the machininginhibit area has been reached when the present position X_(a) coincideswith XL₂, and step (f) comprises deciding that the machining inhibitarea has been passed when the present position X_(a) coincides with XL₁.6. A tracer control method according to claim 4, wherein step (d)comprises generating a number of pulses equivalent to a differencebetween a present position Z_(a) of the machine in the tracing directionand a safe level ZL_(s) when the machining inhibit area is reached, andmoving the machine to the safe level responsive to said pulses.
 7. Atracer control method according to claim 6, wherein step (e) comprisesgenerating a number of pulses equivalent to an interval of the machininginhibit area after the safe level is reached, and causing the machine topass the machining inhibit area responsive to said pulses.
 8. A tracercontrol method according to claim 7, wherein when a resultant deflectionvalue surpasses a predetermined value at the time of an approach, it isdetermined that the approach is complete.
 9. A method of inhibitingtracer machining of a workpiece with a machine tool when an inhibitbegin limit is reached while tracing a model, said method comprising thesteps of:(a) performing tracer machining of the workpiece in dependenceupon the shape of the model until the inhibit begin limit is reached;(b) stopping tracer machining when the inhibit begin limit is reached;(c) moving the machine tool to a safe level when the inhibit begin limitis reached; and (d) moving the machine tool along the safe level afterthe inhibit begin limit is reached.
 10. A method according to claim 9,further including an inhibit end limit and said method furthercomprising the steps of:(e) stopping movement of the machine tool alongthe safe level when the inhibit end limit is reached; (f) approachingthe workpiece with the machine tool when the inhibit end limit isreached; and (g) performing tracer machining of the workpiece independence upon the shape of the model when the approach is step (f) iscompleted.